1. Field of the Invention
The present invention relates to systems using electromagnetic transponders, that is, transmitters and/or receivers (generally mobile) likely to be interrogated contactless and wireless by a unit (generally fixed), called a read and/or write terminal. The present invention more specifically relates to transponders with no autonomous power supply, for example of contactless card or electronic tag type. These transponders extract the power supply required by the electronic circuits included therein from the high-frequency field radiated by an antenna of the read/write terminal. The present invention applies to such transponders, be they read-only transponders, that is, capable of operating with a terminal only reading the transponder data, or read/write transponders which contain data that can be modified by the terminal.
2. Discussion of the Related Art
FIG. 1 very schematically and functionally shows a conventional example of data exchange between a read/write terminal 1 (STA) and a transponder 10 (CAR).
Terminal 1 is essentially formed of an oscillating circuit formed of an inductance L1, in series with a capacitor C1 and a resistor R1, between an output terminal 2p of an amplifier or antenna coupler 3 and a terminal 2m at a reference voltage (generally the ground). Amplifier 3 receives a high-frequency transmission signal Tx, originating from a modulator 4 (MOD). The modulator receives a reference frequency, for example from a quartz oscillator 5 and, if need be, a signal DATA of data to be transmitted. In the absence of any data transmission from terminal 1 to transponder 10, signal Tx is only used as a power source to activate transponder 10 if said transponder enters the field. The data to be transmitted generally originate from a digital system, for example, a microprocessor 6 (μP).
The connection point of capacitor C1 with inductance L1 forms, in the example shown in FIG. 1, a terminal of sampling of a data signal Rx received from a transponder 10, intended for a demodulator (DEM). An output of the demodulator communicates (possibly via a decoder 8 (DEC)) the data received from transponder 10 to microprocessor 6 of terminal 1. Demodulator 7 generally receives from oscillator 5 a clock or reference signal for a phase demodulation. The demodulation may be performed from a signal sampled between capacitor C1 and resistor R1, and not across inductance L1. Microprocessor 6 communicates (BUS) with different input/output circuits (keyboard, screen, means of exchange with a server, etc.) and/or processing circuits. The circuits of the read/write terminal draw the power necessary to their operation from a supply circuit 9 (ALIM) connected, for example, to the electric supply system.
On the side of transponder 10, an inductance L2, in parallel with a capacitor C2, forms a parallel oscillating circuit (called a resonant receive circuit) intended to sense the magnetic field generated by series oscillating circuit L1, C1 of terminal 1. The resonant circuit (L2, C2) of transponder 10 is tuned on the resonance frequency of the oscillating circuit of terminal 1.
Terminals 11 and 12 of resonant circuit L2, C2 which correspond to the terminals of capacitor C2 are connected to two A.C. input terminals of a rectifying bridge 13 having their rectified output terminals 14 and 15 connected across a capacitor Ca of power storage and smoothing of the rectified voltage provided by bridge 13. Bridge 13 is a halfwave or fullwave bridge.
When transponder 10 is in the field of terminal 1, a high-frequency voltage is generated across resonant circuit L2, C2. This voltage rectified by bridge 13 is smoothed by capacitor Ca, which provides a supply voltage to electronic circuits of the transponder via a voltage regulator 16 (REG). These circuits generally comprise, essentially, a microprocessor 17 (μP) associated with a memory not shown, a demodulator 18 (DEM) of the signals possibly received from terminal 1, and a modulator 19 (MOD) for transmitting information to terminal 1. The transponder is generally synchronized by means of a clock (CLK) extracted by a block 20 from the high-frequency signal recovered across capacitor C2 before rectification. Most often, all the electronic circuits of transponder 10 are integrated in a same chip.
To transmit data from transponder 10 to terminal 1, modulator 19 controls a stage of modulation (back modulation) of resonant circuit L2, C2. This modulation stage is generally formed of an electronic switch (for example, a transistor T) and of a resistor R, in series between terminals 14 and 15.
Transistor T is controlled at a so-called sub-carrier frequency (for example, 847.5 kHz), much smaller (generally with a ratio of at least 10) than the frequency of the excitation signal of the oscillating circuit of terminal 1 (for example, 13.56 MHz). When switch T is on, the transponder's oscillating circuit is submitted to an additional damping with respect to the load formed by circuits 16 to 20, so that the transponder draws a more significant amount of power from the high-frequency magnetic field. On the side of terminal 1, amplifier 3 maintains the amplitude of the high-frequency excitation signal constant. Accordingly, the power variation of the transponder translates as an amplitude and current phase variation in antenna L1. This variation is detected by demodulator 7 of the terminal which is either a phase demodulator, or an amplitude demodulator.
In certain cases, the back-modulation stage (transistor T, resistor R) is located upstream of bridge 13, that is, on the side of its A.C. inputs.
The terminal generally does not transmit data while it receives some from the transponder, the transmission occurring alternately in one direction, then in the other.
FIG. 2 illustrates a conventional example of a data transmission from terminal 1 to a transponder 10. This drawing shows an example of the shape of the excitation signal of antenna L1 for a transmission of a code 0101. The modulation currently used is an amplitude modulation with a 106-kilobits-per-second rate (1 bit is transmitted in approximately 9.5 microseconds) much smaller than the frequency (for example, 13.56 MHz) of the carrier originating from oscillator 5 (period of approximately 74 nanoseconds). The amplitude modulation is performed either in all or nothing or with a modulation rate (defined as being the difference of peaks amplitudes (a, b) between the two states (0 and 1) divided by the sum of these amplitudes) smaller than one, due to the need for supply of transponder 10. In the example of FIG. 2, the carrier at 13.56 MHz is modulated, with a rate of 106 kilobits per second, in amplitude with a modulation ratio tm of, for example, 10%.
FIG. 3 illustrates a conventional example of a data transmission from transponder 10 to terminal 1. This drawing illustrates an example of the shape of signal VT of control of transistor T, provided by modulator 19, and of the corresponding signal Rx received by terminal 1. On the transponder side, the back modulation is generally of resistive type with a carrier, called a sub-carrier of, for example, 847.5 kHz (period of approximately 1.18 ms). The back modulation is, for example, based on a BPSK-type coding (binary phase shift keying) with a rate on the order of 106 kilobits per second much smaller than the sub-carrier frequency. In FIG. 3, signal Rx has been shown “smoothed”, that is, without showing the ripple of the high-frequency carrier (at 13.56 MHz). In the example of FIG. 3, it has been considered that each of the three shown bits was different from the previous bit. Thus, a code 010 is being transmitted.
Whatever the type of modulation or of back modulation used (for example, of amplitude, phase, frequency) and whatever the type of data coding (NRZ, NRZI, Manchester, ASK, BPSK, etc.), the modulation is performed digitally, by shift between two binary levels.
As illustrated in FIG. 3, signal VT is formed of a pulse train at the sub-carrier frequency, a phase shift occurring for each state switching from one bit to the next bit.
If several transponders are present in the field of a same terminal, different communications may be initiated between each transponder and the read/write terminal. Most often, the transponders transmit identifiers which enable the terminal to individualize messages respectively intended for them.
In the transponder-to-terminal direction, the transponders determine whether messages are respectively intended for them based on their identifier contained in the message, that they detect after demodulation.
However, a problem may arise when several transponders simultaneously transmit to a same terminal, while they are in the field of this terminal. Such conflicts may be wrongly detected by the read/write terminal, which adversely affects the system reliability.
Further, in some applications, it may be desired for transponders to exchange information. In such a case, the terminal is used as an intermediary for this communication by receiving the information from one transponder to transmit it back to another one, having previously demodulated, then remodulated it.